1. Field of the Invention
This invention relates to drill bits for cutting bore holes in rock or concrete, and more particularly to roof drill bits employed in mining for cutting bore holes in an unsupported mine ceiling so as to permit the securing therein of safety roofing.
2. Background Art
In the field of natural mineral resource extraction, pressure continues unabated for rapid development to meet increasing resource demands and to reduce dependency on imported resources. On the other hand, widespread support exists for the preservation of environmental quality and the protection of the safety of extraction industry workers during the process. These imperatives are nowhere more apparent than in the mining industry.
While mining has in recent years become increasingly automated, speeding resource development and extraction and, to an extent, increasing worker safety, several essential activities in mining have been substantially unaffected by this modernization. In many instances, specific undertakings required of mine workers to advance mine operations have not improved in efficiency and continue to expose workers to substantial risks.
For example, much drilling associated with mining continues to require the immediate presence of mine workers Drilling in any environment is hazardous to a degree, as it involves the use of high speed machinery, the potential for flying debris, and the risk of dust inhalation and noise-induced hearing damage. In addition, however, drilling in mining frequently is required at the sites of the physical expansion of a mine facility. These are dangerous areas, often remote from routes to the surface, where adequate ventilation, structural support, communication facilities, and rescue access means have yet to be installed.
In established areas of a mine, the ceilings are generally roofed over to preclude rock and debris from falling to the floor, injuring workers, or causing hazardous clutter. Such roofing is supported from the mine ceiling itself on rods or bolts which are embedded in a resin in a matrix of vertically oriented bore holes. The bore holes, which extend from two to twenty or more feet (0.61 to 6.1 meters) into the rock of the mine ceiling, must be drilled by using conventional drilling machinery at a point in time when no overhead protection whatsoever can be provided to the drill operator. Only after an entire suitable matrix of bore holes has been created and each filled with appropriate bonding resin to implant rods or bolts therein, can panels of the roofing begin to be mounted on the exposed rod or bolt ends and safety improved. Before this, all workers, and particularly roof drill operators, are exposed to the obvious and grave dangers associated with mine cave-ins. Accordingly, every possible effort is made so that time spent under such conditions is minimized.
In relation to the drilling required to erect roofing, reducing the amount of time it takes to cut each bore hole can contribute to safety by diminishing the time that a drill operator is required to labor beneath an unsupported ceiling of rock. To do so would require increased cutting speed in the tools used which is not without its own difficulties, as will be explained below.
A second way in which the exposure of mining personnel to the risk of unsupported mine ceilings could be diminished would be to reduce the frequency with which cutting tools malfunction. Each breakdown necessitates time spent attending to the drilling tool itself. During these times, mine workers continue to labor under unsafe conditions, and no progress whatsoever occurs in drilling the bore holes themselves. Drilling tools which break down frequently or which are complicated to repair or replace only expose the unlucky operator to longer periods of risk, uncertainty, and possible injury.
In addition, once an adequate matrix of roof support bore holes has been drilled, it is important that the resin used to hold the roofing support rods or bolts will securely adhere to the sides of the bore holes. Where the drilling techniques employed to produce the bore holes result in bore hole walls that bear an accumulation of drilling cuttings, reliable resin bonding is unpredictable. Unclean bore holes, thus exacerbate the dangers involved in opening new areas of a mine by giving rise to the possibility that alternate bore holes may need to be drilled, or worse, that installed roofing may give way.
In reducing the frequency and severity of mechanical failures in roof drills and in improving the cleanness of completed bore holes, it is critical to focus on the drill bit employed. During use, the drill bit is forced against the end of the bore hole and rotated at high speed in a confined area. These conditions give rise to high temperatures and severe mechanical stresses in the drill bit. Efforts to speed drilling by running machinery faster will ultimately prove counterproductive if by increasing the heat and stress to which the drill bit is exposed, higher rates of failure and increased down time result.
The cuttings dislodged from the end of a bore hole by the cutting member of a drill bit must be removed from the bore hole, if drilling is to progress and if equipment failure is to be avoided. Clearing cuttings from the bore hole permits the drill bit to be advanced so as to continue to cut out new rock. Once removed from the end of the bore hole, cuttings must thereafter eventually exit the bore hole entirely
Behind the drill bit, substantially filling the cross-section of the bore hole, are substantially cylindrical, usually hollow shaft-like drill steels with which the drill bit is driven. If cuttings are not removed, constriction and friction in the mechanical assembly will intensify, a problem that is exacerbated by the known tendency of mining cuttings to expand upon being dislodged. This is due to the fact that the rock from which the cuttings are dislodged has itself long been in a state of compression arising from the weight of overlying strata of rock and the mechanical geological activity in the earth.
In addition, unless cuttings are effectively and promptly removed from the vicinity of the rotating drill bit and eventually from the bore hole itself, conditions of heat and temperature can develop in which drillings will fuse together on the drill bit and other parts of the assembly, as well as against walls of the bore hole. The latter reduces the effective size of any resulting bore hole and contributes to a bore hole which does not bond well with the resin inserted to hold roof support rods.
One approach to removing cuttings from the end of a bore hole has been to use air suction to draw the cuttings out of the end of the bore hole through passageways in the drill bit that communicate with the interior of the associated drill steels. This process, however, produces an unhealthy high level of dust in the air in a mine, and has begun to be viewed with disfavor.
Alternatively, in order to both cool the drill bit and attempt to wash cuttings from the end of the bore hole, water has been introduced into the end of the bore hole through the interiors of drill the steels and various patterns of communicating passageways formed in the drill bit. Such efforts, however, have not to date been entirely satisfactory due to the failure to discover a combination of passageway positioning and drill bit shape that will reliably and rapidly remove cuttings and water from the end of the bore hole. Frequently, cuttings still accumulate on the drilling assembly or the walls of the bore hole.
A particular problem is encountered when the bore hole passes through softer stratas of rock, sand, or clay. These substances are also under great pressure. Being somewhat plastic in comparison with other rock, they will frequently flow out of the space in which they are confined and into a bore hole when that space is penetrated by an advancing drill bit. This sudden flood of material can overwhelm the capacity of a cuttings extraction system. Temperature and pressure will build, and localized regions of the bore hole will dry up, fusing cuttings and causing fluid outflow to become constricted and channeled into localized areas around the bore hole wall, if not cut off totally.
The water required to remove cuttings in such systems raises additional difficulties. A mine is an environment in which the volume of space is relatively limited. Water used in a mine for any purpose can become a problem if its volume is substantial. In light of environmental regulations, it is desirable not to bring water used for the purpose of flushing cuttings from bore holes to the surface after its use. Therefore, the water utilized to cool and flush such drilling operations should be of as small a volume as possible so as to permit its storage and processing underground in the mine itself. Slowly cutting drills will draw flush water for longer periods of time, suggesting an additional advantage to fast bore hole drilling. Nevertheless, fast drilling results in greater heat and a more rapid generation of cuttings. This in turn may require a more voluminous flow of water. Where water for this purpose cannot be located underground within a mine, it must be introduced into the mine from outside, presenting an additional difficulty for the mining engineer.
Therefore, it can be seen that the need remains unfulfilled for the development of a drill bit of maximum durability which will cut rock at the fastest rate possible and which will withstand the temperature and mechanical stresses associated with the environment in which it is used. The durability of such a drill bit head can be enhanced through the design of an optimal water flow system for removing cuttings from the end of the bore hole and cooling the drill bit itself.
A drill bit for use under such circumstances would be further desirable if designed in such a fashion as to enable its rapid and easy replacement if and when it should malfunction. Also, a drill bit head that reliably produces bore holes with clean and readily bondable walls would further reduce the need for redrilling and shorten the exposure of workers to dangerous conditions.
Until the development of the present invention, however, a drill bit meeting these requirements of the modern mining industry was not available.